Ed Miliband delivers his speech on Europe at the London Business School. Photograph: Andy Rain/EPA

Britain's leading business organisations are divided over Ed Miliband's pledge to offer an in-out referendum on Britain's EU membership if UK sovereignty is ceded to the EU in any future treaty revision.

As David Cameron accused the Labour leader of designing a policy by committee that would offer no choice and no referendum, the Confederation of British Industry praised Miliband for giving businesses certainty.

But the Institute of Directors said a referendum needed to be held, as it called on Labour to do more to set out its plans to make the EU more competitive.

The main business organisations spoke out as Miliband told the London Business School that Labour would reject the "threat mechanism" proposed by the prime minister to hold an in-out referendum on Britain's EU membership by 2017 after negotiations to reform the EU. Miliband will instead offer a referendum only if UK powers are ceded to Brussels.

Miliband denied that his plans were unclear after the FT reported that he was knocking down an early referendum while the Daily Mirror said he was holding out the prospect of a vote. Asked by the BBC's Nick Robinson what he would say to a voter who asked for a guarantee that they would be given a say on the EU, Miliband said: "If there is a transfer of powers from Britain to the EU, then there will be an in-out referendum. My priority for the next parliament, though, is tackling the cost of living crisis, protecting the NHS and jobs for our young people."

The prime minister dismissed Miliband's conditional promise. Speaking on the plane to Israel, Cameron said: "This is not a policy for an in/out referendum. This is a policy clearly designed by a committee that obviously could not agree what to do, and have come up with a policy that makes no sense whatsoever."

He added: "The British people now have a very, very clear choice. At the next election, they can elect a Tory government that will renegotiate Britain's position in Europe and give people a guaranteed in/out referendum by the end of 2017. If they get a Labour government they get no referendum, no choice, nothing.

"It could not be clearer when you read that article. It was a bit confused overnight in their attempted briefing. It looked like some newspapers swallowed that briefing.

"If you read that Financial Times article it is absolutely clear, if you get Miliband, you don't get a referendum, you don't get a choice. It's the classic Labour 'we know best'."

Downing Street was delighted with the revised Labour stance claiming Miliband had provided them with a big stick to beat him with between now and polling day.

One source said: "It feels like they thought they had for them made a big move, but in reality out there with the public this will mean nothing."

They pointed repeatedly to Miliband's own assessment that it was unlikely there would be any proposed transfer of power to Brussels in the next parliament to trigger a referendum.

"That means no choice between 2015 and 2020," the source said.

But Miliband dismissed No 10's argument that the "threat mechanism" of an in-out referendum after the next election – even if no UK powers are transferred to the EU – was the best way of protecting British interests in the single market when new eurozone governance arrangements are negotiated.

The Labour leader said: "It is a really simple point. Fifteen months ago the jury was out on whether David Cameron's strategy was going to work – the so-called threat mechanism. It has totally failed. He is totally marginalised. He is totally irrelevant. He has marginalised himself and won no friends.

"If it was the case that this great threat mechanism would have had a really positive effect for our country I think we would have seen it by now. Actually what you had was Angela Merkel coming along to make that very good speech she gave in Westminster to both houses of parliament saying very clearly: we are not going to have a part with this. We have got no support for this. We are far more likely to achieve reform in Europe with the approach I have outlined."

Sir Mike Rake, the CBI president, said: "Business will welcome Labour's decision to make its policy position on Europe clear. Any uncertainty is unhelpful when trying to secure long-term investment.

"The CBI strongly supports Ed Miliband's view that we are better off in a reformed EU than outside with no influence. We must take a sensible approach to building alliances to get the reforms we need.

"Calling a referendum is a matter for politicians to decide. But business believes that future investment, growth and jobs depend on the UK being part of a competitive and outward-looking EU."

Simon Walker, director general of the IoD, said: "The European Union must adapt in order to survive, and we welcome Labour's efforts to be part of the debate on the future of the UK's membership. However, to be taken seriously they must set out exactly how they plan to make the EU more competitive, more accountable and more relevant. All efforts must be focused on ensuring that Europe's businesses can flourish, creating jobs and prosperity across the continent.

"Key to meaningful reform is liberalising employment and company law. Ed Miliband supports giving back more control to national parliaments, and for the UK this must include control over social legislation like working hours and agency workers. The majority of IoD members think the EU's interventions in this area have been unhelpful.

"Miliband says he would not want a Labour government to be deflected from its economic agenda by pursuing an in-out referendum in 2017, but Britain's economic interests are intricately bound up in the debate surrounding EU reform. IoD members are pragmatic, there is little appetite for withdrawal, but there is broad support for reform of its institutions and practices. They recognise that the EU has to change, and it makes sense to put such changes to the British people."